National Portrait Gallery boss Sandy Nairne is stepping down from the job after 12 years in charge.
Nairne, who is leaving the central London gallery in February, said it had been "a great privilege to lead such a special institution".
He said: " The fact that two million visitors now come each year to visit exhibitions, take part in activities or see displays of this amazing collection in London, as well as around the country or online, is testimony to the dedication of all who work at the gallery and those who support it in so many different ways. The gallery is in very good shape and will go from strength to strength."
The gallery said Mr Nairne, who has also worked at the Arts Council and Tate, is leaving to " pursue his writing and advisory work".
In his time in charge, the gallery has seen annual attendance figures rise by more than 500,000 to two million and launched its largest public appeal that raised more than £10 million to keep a self-portrait of painter Sir Anthony van Dyck in the country.
Sir William Proby, who chairs the gallery's trustees, said he had "done a tremendous job".
He said: " He has significantly increased visitor numbers, put on some wonderful exhibitions, such as Lucian Freud Portraits, and overseen many major commissions and acquisitions, including the Van Dyck self-portrait this year.
"He has built a very strong team and prepared them well for the future. On behalf of all my fellow trustees, I would like to thank him for his exemplary leadership and wish him well for the future."